1. Field of This Invention
This invention relates to a process for the production of pigmented or nonpigmented self-cross-linking lacquers utilizing an aqueous dispersion.
2. Prior Art
The hitherto customary coatings for metals have been based on the use of a drying oil and of a fast drying lacquer base substance (such as, natural resins, a combination of natural and synthetic resins or of an alkyd resin modified with a urea, melamine or phenol formaldehyde resin) -such contains volatile organic solvents which frequently are flammable and which during the coating process develop harmful vapors. Because of this, special measures are needed for the recapture of the solvents themselves. Other proposed binders for coatings contain unsaturated polymer systems, such as butadiene-styrene lattices. However, this polymer type is imbued with the known disadvantages of becoming brittle and capable of discoloration upon progressive oxidation.
There has been a need for a long time for coating compounds which can be applied from aqueous media and consequently do not develop any undesirable flammable or toxic vapors, and which are self-cross-linking solely as a result of heat action.
Known aqueous systems are described in the U.S. Pat. Nos. 2,760,886, 2,918,391 and 3,033,811. These coating masses are not satisfactory since they are inclined to dry prematurely during application because they are not able to form a smooth surface film due to the formation of blisters during the heat treatment.
Heat hardenable coating compounds in the form of aqueous lattices are also known (see German published application No. 2,211,169). Such are produced in accordance with the customary emulsion polymerization process. The binders used in such are based on two compounds, one being an additional copolymer consisting of an aromatic monovinyl monomer, an ester of the acrylic or methacrylic acid, at least one olefinic unsaturated monomer with an amide or hydroxyl groups and one olefinically unsaturated monomer with a carboxyl group, and the other consisting of a water soluble urea-formaldehyde resin. The disadvantage of these coating compounds lies in the fact that they are not self-curing (self-cross linking) and thus a multiple of important characteristics and properties (such as, pigment compatability, distribution of the pigments, adhesive capacity, luster, resistance to water and solvents, resistance to heat and atmospheric corrosion) are impaired through the addition of a considerable amount of a second cross-linking component.